In fairness to Fallout, the desert aesthetic wasn't originally due to the fallout and radiation, but rather because the original game was set in the Southwest US which is... well...
Here's RL New Reno, for instance:
Or here's the real world town near Junktown from Fallout 1, Ridgecrest CA:
Or the real life location near the Hub, Lancaster CA:
Now, granted, Fallout 2 continued that aesthetic even in areas where it arguably shouldn't have, for instance Yreka California, which is near to where Klamath is in FO2 looks like this IRL:
However, both Fallout 2 and Fallout 3 continued the desert aesthetic in regions they shouldn't have. I'll grant more leeway to Fallout 2, as it is an older game and they might have been attempting to save development time and disk space by limiting them number of environmental backgrounds they had to design, but there's no excuse for Fallout 3... Why? Well, here's Rockville Maryland, Bethesda's HQ IRL and a suburb of Washington DC:
Or, to be more explicit, Fairfax Virginia has ruins in Fallout 3... and is also a city in real life. The real life city looks like this:
Basically, they established a desert aesthetic in the first game, and stuck to it even though they moved into areas where it made no sense, neither northern California (in FO2) or the DC Metro Area (FO3) are even remotely desert-like (heck, the DC Metro area is only a few inches per annual rainfall of being a Temperate Rainforest).